Many variations of collets have evolved over the years, however, the most commonly used collet is one which has a tubular body portion and a plurality, usually three or four, spring fingers extending axially from the body portion. On the exterior of the distal end of the spring fingers is formed a cam surface, while on the interior is formed a work gripping surface. The collet fits into the spindle of a lathe or other machine tool, and the spindle has a tapered cam surface which coacts with the cam surface on the collet such that relative axial movement between the collet and the spindle will cause flexing of the spring fingers and thus an opening or closing of the collet. Such types of collets are illustrated in U.S. Pat. No. 2,557,301.
Another commonly used collet is similar to that previously discussed but uses replaceable work gripping pads secured within the collet. The pads may have any desired work gripping configuration such as round (of any diameter), hexagonal, square, polygonal, or irregular shape, so that a great variety of work may be accommodated within a single collet merely by changing the pads. Such a construction is illustrated in U.S. Pat. No. 2,234,603.
The collet opening or closing action and the resultant flexure of the spring fingers is repeated rapidly in modern machines and in the high speed production-type lathes, the opening and closing cycles occur quite frequently. A frequent problem with such collets in these machines is that breakage occurs in the leaf section of the spring fingers, immediately behind the cam surface. The possibility of such breakage is accentuated when a short workpiece is gripped at the front of the collet, i.e., is not inserted very far into the collet. This breakage renders the entire collet useless. Additionally, if the breakage occurs during a machining operation, the workpiece would probably be damaged, and a safety hazard may be created by a flying workpiece or collet part.
An additional possible contributing factor to the breakage in conventional collets is the heat treatment necessarily used. The head end of the collet must be heat treated to harden it for wear resistance, whereas the body of the collet must be less hard in order to achieve the requisite spring tension. Thus, the two heat treatments are to a certain extent conflicting and may cause metallurgical stresses which contribute to weakness near the juncture of the head and the body of the collet.
Accordingly, a primary object of this invention is to overcome the disadvantages of prior art collets.
Another object of this invention is to provide a collet assembly which utilizes a spring body member and separate but cooperating work gripping jaws.
Another object of this invention is to provide a spring body member which is replaceable without replacing the entire collet assembly.
A further object of this invention is to provide work gripping pads which has the cam actuating surface and work gripping surface thereon.
Still a further object of this invention is to provide a collet assembly capable of accurate workpiece centering.
Still a further object of this invention is to provide a collet assembly wherein the work gripping pads having the cam actuating surface and work gripping surface thereon are removably attached to the spring body.
A further object of this invention is to provide a collet wherein the spring tension of the spring body opens the collet to release the workpiece when the actuator sleeve is released.
Yet another object of this invention is to provide a collet assembly wherein the head and body portions may be separately heat treated without introducing conflicting side effects.